In recent years, it was reported that human interferon-α (may be called “IFN-α”, hereinafter) subtype α8 (may be simply called “IFN-α8”, hereinafter) exhibits a superior activity to other IFN-α subtypes, accepted as pharmaceuticals, such as subtypes α2a and α2b (may be respectively called “IFN-α2a” and “IFN-α2b”, hereinafter). For example, Foster G. R., Rodrigues O., Ghouze F., Schulte-Frohlinde E., Testa D., Liao M. J., Stark G. R., Leadbeater L., and Thomas H. C. reported in Journal of Interferon & Cytokine Research, Vol. 16, No. 12, pp. 1027-1033, 1996, that IFN-α8 shows an extremely higher anti-viral activity than other IFN-α subtypes. Yanai Y., Horie S., Yamamoto K., Yamauchi H., Ikegami H., Kurimoto M., and Kitamura T. reported in Journal of Interferon & cytokine Research, Vol. 21, No. 12, pp. 1129-1136, 2001, that IFN-α8 exhibits a superior anti-tumor activity on kidney cancer to other IFN-α subtypes.
The above reports show that IFN-α8 has a superior activity to other IFN-α subtypes, however, the fact was found based on the results only from in vitro experiments and the difference in activity between IFN-α8 and other IFN-α8 subtypes is not so distinct. Therefore, if only mutant IFN-α8 proteins having a much higher activity than conventional IFN-α preparations or IFN-α8 were obtained, they would expectedly expand the use of IFN-α.